This invention relates to electrostatic spray guns, and in particular a method for replacing spent dielectric material in the barrel of an electrostatic spray gun to reduce corona discharge therein.
Conventional electrostatic spray guns project fluid coating material such as paint, varnish, lacquer and the like in atomized or particulate form toward an object to be coated. The object to be coated is held at electrically ground potential and an electric charge is imparted to the coating material so that it will be electrostatically attracted to the object. In order to assure that a high percentage of the coating material ejected from the electrostatic spray gun is deposited on the object, high charging voltages, up to 120 kv, are typically applied to the coating material.
When spraying many of the coating materials in use today, including powders, a readily ignitable atmosphere results in the area of the coating operation. Energizing the high voltage electrostatic charging circuit associated with the spray gun causes energy to be capacitively stored in the electrically conductive components of the charging system. If the gun is brought too close to any grounded object, the possibility arises that a spark will jump between the high voltage circuit in the gun and the grounded object igniting the flammable atmosphere in the coating area. Many recent improvements in electrostatic spray guns have been directed to reducing incendevity resulting from the discharge of capacitively stored electrical energy, such as disclosed, for example, in U.S. Pat. Nos. 4,182,490; 4,241,880; 4,273,293; and 4,335,851, all assigned to the same Assignee as this invention.
One means of damping discharge of electrical energy capacitively stored in the charging circuit, disclosed in the prior patents mentioned above, is the provision of a high-value resistor in the barrel of the gun. The resistor is interposed between a high voltage cable carrying the electrical charge from a source, and a high voltage lead which communicates with the nozzle assembly of the gun in which the coating material is charged. The high-value resistor effectively reduces energy capacitively stored in the gun barrel when properly insulated.
It has been found, however, that if air is present in the area of the connection between the resistor and high voltage lead, a corona discharge can occur which attacks the high voltage connection and surrounding housing of the gun barrel. In order to eliminate air around the high voltage connection between the resistor and lead, it has been the practice in the prior art to completely cover the resistor with a dielectric material such as grease. According to this method, the resistor is first encapsulated with grease and then inserted into a bore formed in the gun barrel into contact with the high voltage lead. In some instances, additional grease is placed in the gun barrel bore prior to insertion of the resistor. This method of eliminating air gaps around the resistor and providing an uninterrupted layer of nonconductive grease between the resistor and the gun barrel, and around the high voltage connection between the resistor and lead, has not been entirely effective.
An improved method of completely insulating the resistor and its high voltage connections with dielectric material is disclosed in the above-mentioned co-pending U.S. patent application.
In accordance with the method of that invention, an insulating tube is inserted within a bore formed in the electrostatic gun barrel. The outside diameter of the tube is slightly less than the inside diameter of the bore so as to form a gap therebetween. A high voltage lead is disposed at the inner end of the bore, and a fitting is connected to the gun barrel at the outer end of the bore. A high-value resistor having a compliant contact such as a spring at one end, is inserted into the insulating tube so that it contacts the high voltage lead. The resistor and spring are not covered with grease at this point, and are surrounded with air contained in the insulating tube. A quantity or slug of flowable, dielectric material such as grease is then deposited in the insulating tube at its outer end adjacent the fitting. A high voltage cable, which carries the electrical charge to be applied to the coating material, is next inserted through the fitting and into contact with the grease in the insulating tube. The cable tightly fits within the insulating tube so that it pushes the grease ahead toward the resistor and spring. The cable is advanced until its leading end contacts the spring which urges the resistor against the high voltage lead. The cable is then connected to the fitting to retain it in place within the insulating tube.
The grease is forced by the cable over and around the spring and resistor, completely encapsulating both elements. A sufficient quantity of grease is provided so that the spring and resistor are entirely encapsulated and at least a portion of the grease is forced into the gap between the insulating tube and bore in the gun barrel. This assures that all of the air in the area of the resistor is forced ahead of the grease into the gap between the insulating tube and bore, away from the resistor. The grease effectively insulates the resistor and its connection to the high voltage lead to eliminate corona discharge thereat.
Typically, the insulating tube, resistor and grease are inserted in the gun barrel by the manufacturer prior to sale. The customer then inserts the high voltage cable into the insulating tube at the manufacturing site in preparation for a coating operation. From time to time the cable is removed to reposition the gun or perform maintenance and then later reinserted to begin another production run.
While the above-described method taught in U.S. patent application Ser. No. 539,087 mentioned above has proved to be effective in encapsulating the resistor with an initial slug of grease, difficulties have been encountered in replacing the spent grease with new grease when the cable is reinserted into the insulating tube. If too much or too little new grease is injected into the insulating tube, air gaps are formed in the area of the resistor causing corona discharge to occur. Replacement of spent grease which is initially placed on the resistor directly prior to insertion into the gun, as practiced in other prior art methods, also presents problems of creating unwanted air gaps in the vicinity of the resistor.